1. Field of the Invention
The present invention relates to wireless terminal devices and, more particularly to a wireless terminal device, such as a portable telephone, having a receiving unit with a direct conversion system in which a received signal is converted directly into a base band signal.
2. Description of the Background Art
In a heterodyne conversion system for receivers, which has been well known, a received signal is converted into an intermediate frequency (IF) signal by a mixer (MIX) and then an unwanted wave is removed by a band-pass filter (BPF) for the IF. After that, complex envelope detection is performed by a quadrature mixer (QMIX) to supply a base band signal. In the heterodyne conversion system, however, as the intermediate frequency exists an intermediate frequency circuit, such as a band-pass filter, must be provided, resulting in the increase in the number of components and space for mounting the components.
On the other hand, in a direct conversion system, a received signal is directly converted into a base band signal by a quadrature mixer. FIG. 1 is a block diagram referenced for describing the direct conversion system.
FIG. 1 shows an example in which the direct conversion system is employed for a portable telephone which operates according to a certain communication system, such as CDMA system, in which transmission and reception are performed simultaneously and, a transmission system connected to a duplexer (DUP) 2 is not shown.
In FIG. 1, a high-frequency signal received by an antenna 1 is supplied to a low-noise amplifier (AMP) 3 via duplexer 2, a received signal of a low level is amplified, an unwanted high-frequency band component such as an external interference wave is removed by a band-pass filter (BPF) 4 and the remaining component is converted into a base band signal by a quadrature mixer (QMIX) 5.
Here, the relation between a frequency frx of the received signal, which is an output from band-pass filter 4 and a local oscillation frequency flo of quadrature mixer 5 is represented by the following expression:
fbb=frxxe2x88x92fro,
where fbb is the base band signal. As the frequency of the base band signal is sufficiently lower than that of the received signal, frx=flo holds.
The base band signal obtained through conversion is amplified by a low-noise amplifier (AMP) 6 and the resulting signal is supplied to a low-pass filter (LPF) 7 where an interference wave such as an adjacent channel component is removed from the base band signal. Further, the base band signal is supplied to a variable gain amplifier (VGA) 8 and amplified to maintain an input to an A/D converter 9 in the next stage at a constant level and, the resulting signal is converted into a digital signal by A/D converter 9 and, demodulated at a demodulation circuit (DEM) 10.
In the receiver of the direct conversion system shown in FIG. 1, the elimination of the mixer and the band-pass filter for the intermediate frequency required in the heterodyne conversion system is allowed to achieve a simple circuit structure. In addition, a spurious response of the receiver to a signal represented by an image signal can be mitigated whereby the high-frequency band-pass filter can be replaced with a smaller band-pass filter 4.
Further, a dynamic range of a receiver adapting the heterodyne system is controlled such that the gain of the variable gain amplifier becomes variable in an intermediate frequency portion and an input signal to a base band portion becomes fixed.
On the other hand, a receiver adapting the direct conversion system generally does not include an intermediate frequency portion having a variable gain amplifier, and hence, at the fluctuation of the input signal, if the gain is not changed accordingly at a high frequency portion, the level of an input signal to an analog base band portion including low-noise amplifier 6, low-pass filter 7 and variable gain amplifier 8 in FIG. 1 fluctuates.
Particularly, when a signal of a high range is to be input, an element with a wide dynamic range must be employed as the analog base band portion. In the direct conversion system, however, the use of the element with a wide dynamic range is more difficult than in the heterodyne system which has means for amplifying a gain at the intermediate frequency portion, because, in the direct conversion system, the intermediate frequency portion performing amplification with a very high gain is not provided and, very high gain, very low noise, and very good distortion characteristic are required in the analog base band portion. When these requirements are met and a wide dynamic range is achieved, if means for varying gain is provided in a path to the analog base band portion, saturation or distortion at the amplifier or the low-pass filter of the analog base band portion must be prevented at the input of a signal with a high level.
On the other hand, the above mentioned problems can be obviated by switching the gain at amplifier 3 in a high frequency stage. However, the accuracy of the gain control at amplifier 3 is hard to secure. Because a factor which is hard to be dealt with in the design consideration, such as parasitic capacitance by the high frequency, becomes noticeable and also because increase in the process variation makes it difficult to secure the accuracy of the gain. Further, because the amplifier employed in the high frequency portion often has unstable temperature characteristic. In addition, it is difficult to switch the gains of a plurality of stages at the high frequency stage with high accuracy.
Hence, a main object of the present invention is to provide a wireless terminal device wherein the accuracy of the gain control can be maintained and saturation at the base band portion can be prevented.
The present invention is a wireless terminal device directly converting a high frequency signal into a base band signal, wherein the base band signal generated by conversion is supplied to and amplified at an amplifying circuit 2, an interference wave signal component is removed from an output signal therefrom through a filter and the resulting signal is supplied to a variable gain circuit and, the level of the signal supplied to the variable gain circuit is detected, the gain of the amplifying circuit is switched based on a detection output and the gain of the variable gain circuit is controlled.
According to another aspect of the present invention, the amplification circuit has a variable gain and, the control circuit changes the gain of the amplification circuit based on the detection output of the level detection circuit.
According to still another aspect of the present invention, the amplification circuit includes a plurality of amplifiers with different gains, and a select circuit selecting one of the plurality of amplifiers based on a control signal.
According to still another aspect of the present invention, power supply to the amplifying circuit is cut off based on a control signal supplied from the control circuit to prevent the appearance of a signal of a high level on an output from the amplifying circuit.
According to still another aspect of the present invention, the amplification circuit, the filter, the variable gain circuit and the level detection circuit are arranged on the same integrated circuit and the integrated circuit is provided with a gain control circuit setting the gain of the amplification circuit based on a clock signal and data supplied from the control circuit.
According to still another aspect of the present invention, the control circuit supplies data on the gain to be set to the amplification circuit in synchronization with the clock signal in a time divisional manner.
According to still further aspect of the present invention, the gain control circuit switches each of the gain of the amplification circuit in a step-like manner.
The foregoing and other objects features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.